Surface acoustic wave (saw) resonator

ABSTRACT

A surface acoustic wave (SAW) resonator includes a piezoelectric layer disposed over a substrate, and a plurality of electrodes disposed over the first surface of the piezoelectric layer. A layer is disposed between the substrate and the piezoelectric layer. A surface of the layer has a smoothness sufficient to foster atomic bonding between layer and the piezoelectric layer. A plurality of features provided on a surface of the substrate reflects acoustic waves and reduce the incidence of spurious modes in the piezoelectric layer.

BACKGROUND

Electrical resonators are widely incorporated in modern electronicdevices. For example, in wireless communications devices, radiofrequency (RF) and microwave frequency resonators are used in filters,such as filters having electrically connected series and shuntresonators forming ladder and lattice structures. The filters may beincluded in a duplexer (diplexer, triplexer, quadplexer, quintplexer,etc.) for example, connected between an antenna (there could be severalantennas like for MIMO) and a transceiver for filtering received andtransmitted signals.

Various types of filters use mechanical resonators, such as surfaceacoustic wave (SAW) resonators. The resonators convert electricalsignals to mechanical signals or vibrations, and/or mechanical signalsor vibrations to electrical signals.

While certain surface modes are desired, certain standing spurious modescan exists between the opposing faces of the piezoelectric material ofthe SAW resonator. These spurious modes are parasitic, and can impactthe performance of filters comprising SAW resonators.

What is needed, therefore, is a SAW resonator structure that overcomesat least the shortcomings of known SAW resonators described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion. Wherever applicable and practical, like referencenumerals refer to like elements.

FIG. 1A is a top view of a SAW resonator structure according to arepresentative embodiments.

FIG. 1B is a graph of admittance versus frequency.

FIG. 1C is the cross-sectional view of a SAW resonator structure of FIG.1A along line 1C-1C.

FIG. 1D is a cross-sectional view of a portion of the SAW resonatorstructure of FIG. 1C.

FIG. 1E is a cross-sectional view of a portion of a SAW resonatorstructure in accordance with a representative embodiment.

FIG. 1F is a cross-sectional view of a portion the SAW resonatorstructure of FIG. 1C.

FIG. 2 is a flow-chart of a method of fabricating a SAW resonatorstructure according to a representative embodiment.

FIG. 3 is a simplified schematic block diagram of a filter comprising aSAW resonator structure according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, representative embodiments disclosing specific detailsare set forth in order to provide a thorough understanding of thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparatuses andmethods may be omitted so as to not obscure the description of therepresentative embodiments. Such methods and apparatuses are clearlywithin the scope of the present teachings.

It is to be understood that the terminology used herein is for purposesof describing particular embodiments only, and is not intended to belimiting. Any defined terms are in addition to the technical andscientific meanings of the defined terms as commonly understood andaccepted in the technical field of the present teachings.

As used in the specification and appended claims, the terms ‘a’, ‘an’and ‘the’ include both singular and plural referents, unless the contextclearly dictates otherwise. Thus, for example, ‘a device’ includes onedevice and plural devices.

As used in the specification and appended claims, and in addition totheir ordinary meanings, the terms ‘substantial’ or ‘substantially’ meanto with acceptable limits or degree. For example, ‘substantiallycancelled’ means that one skilled in the art would consider thecancellation to be acceptable.

As used in the specification and the appended claims and in addition toits ordinary meaning, the term ‘approximately’ means to within anacceptable limit or amount to one having ordinary skill in the art. Forexample, ‘approximately the same’ means that one of ordinary skill inthe art would consider the items being compared to be the same.

Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and“lower” may be used to describe the various elements' relationships toone another, as illustrated in the accompanying drawings. These relativeterms are intended to encompass different orientations of the deviceand/or elements in addition to the orientation depicted in the drawings.For example, if the device were inverted with respect to the view in thedrawings, an element described as “above” another element, for example,would now be “below” that element. Similarly, if the device were rotatedby 90° with respect to the view in the drawings, an element described“above” or “below” another element would now be “adjacent” to the otherelement; where “adjacent” means either abutting the other element, orhaving one or more layers, materials, structures, etc., between theelements.

In accordance with a representative embodiment, a SAW resonatorstructure comprises a substrate having a first surface and a secondsurface. The first surface of the substrate has a plurality of features.A piezoelectric layer is disposed over the substrate. The piezoelectriclayer has a first surface and a second surface. A plurality ofelectrodes is disposed over the first surface of the piezoelectriclayer, and the plurality of electrodes is configured to generate surfaceacoustic waves in the piezoelectric layer. The SAW resonator structurealso comprises a layer disposed between the first surface of thesubstrate and the second surface of the piezoelectric layer, the firstsurface of the layer having a smoothness sufficient to foster atomicbonding between the first surface of the layer and the second surface ofthe piezoelectric layer, wherein the plurality of features reflectacoustic waves back into the piezoelectric layer.

FIG. 1A is a top view of a SAW resonator structure 100 according to arepresentative embodiment. Notably, the SAW resonator structure 100 isintended to be merely illustrative of the type of device that canbenefit from the present teachings. Other types of SAW resonators,including, but not limited to a dual mode SAW (DMS) resonators, andstructures therefor, are contemplated by the present teachings. The SAWresonator structure 100 of the present teachings is contemplated for avariety of applications. By way of example, and as described inconnection with FIG. 3, a plurality of SAW resonator structures 100 canbe connected in a series/shunt arrangement to provide a ladder filter.

The SAW resonator structure 100 comprises a piezoelectric layer 103disposed over a substrate (not shown in FIG. 1A). In accordance withrepresentative embodiments, the piezoelectric layer 103 comprises on oflithium niobate (LiNbO₃), which is commonly abbreviated LN; or lithiumtantalate (LiTaO₃), which is commonly abbreviated LT.

The SAW resonator structure 100 comprises an active region 101, whichcomprises a plurality of interdigitated electrodes 102 disposed over apiezoelectric layer 103, with acoustic reflectors 104 situated on eitherend of the active region 101. In the presently described representativeembodiment, electrical connections are made to the SAW resonatorstructure 100 using the busbar structures 105.

As is known, the pitch of the resonator electrodes determines theresonance conditions, and therefore the operating frequency of the SAWresonator structure 100. Specifically, the interdigitated electrodes arearranged with a certain pitch between them, and a surface wave isexcited most strongly when its wavelength λ is the same as the pitch ofthe electrodes. The equation f₀=v/λ describes the relation between theresonance frequency (f₀), which is generally the operating frequency ofthe SAW resonator structure 100, and the propagation velocity (v) of asurface wave. These SAW waves comprise Rayleigh or Leaky waves, as isknown to one of ordinary skill in the art, and form the basis offunction of the SAW resonator structure 100.

Generally, there is a desired fundamental mode, which is typically aLeaky mode, for the SAW resonator structure 100. By way of example, ifthe piezoelectric layer 103 is a 42° rotated LT, the shear horizontalmode, having a displacement in the plane of the interdigitatedelectrodes 102 (the x-y plane of the coordinate system of FIG. 1A). Thedisplacement of this fundamental mode is substantially restricted tonear the upper surface (first surface 110 as depicted in FIG. 1C) of thepiezoelectric layer 103. It is emphasized that the 42° rotated LTpiezoelectric layer 103, and the shear horizontal mode are merelyillustrative of the piezoelectric layer 103 and desired fundamentalmode, and other materials and desired fundamental modes arecontemplated.

However, other undesired modes, which are often referred to as spuriousmodes, are established. Turning to FIG. 1B, a graph of admittance versusfrequency is depicted for the illustrative 42° rotated LT piezoelectriclayer 103. The desired fundamental mode, the shear horizontal mode 106,is substantially restricted to the upper surface of the piezoelectriclayer 103, and has a frequency at series resonance (F_(s)). However, anumber spurious modes 107, having frequencies greater than the frequencyat parallel resonance (F_(p)), can exist in the piezoelectric layer 103.As described more fully below, these spurious modes 107 are created byacoustic waves generated in the piezoelectric layer 103 that establishstanding wave of various kinds of modes (with different modal shapes andfrequencies). More specifically, these spurious modes 107 are created byreflections at the interface of the piezoelectric layer 103 and thesubstrate (see FIG. 1C) of the SAW resonator structure 100.

The spurious modes can deleteriously impact the performance of SAWresonators, and devices (e.g., filters) that include SAW resonators, ifnot mitigated. Most notably, if a first filter is comprised of one ormore SAW resonators, and is connected to a second filter having apassband that overlaps the frequency of the spurious modes, a sharpreduction in the quality (Q) of the second filter will occur. Thespurious modes are observed on a so-called Q-circle of a Smith Chart ofthe S₁₁ parameter. These sharp reductions in Q-factor are known as“rattles,” and are strongest in the southeast quadrant of the Q-circle.Beneficially, significant mitigation of the adverse impact of thesespurious modes is realized by the various aspects of the presentteachings as described below.

FIG. 1C is a cross-sectional view of the SAW resonator structure 100depicted in FIG. 1A along the lines 1B-1B. The SAW resonator structure100 comprises a substrate 108 disposed beneath the piezoelectric layer103, and a layer 109 disposed between the substrate 108 and thepiezoelectric layer 103.

As noted above, the piezoelectric layer 103 illustratively comprises oneof LN or LT. Generally, in the representative embodiments describedbelow, the piezoelectric layer 103 is a wafer that is previouslyfabricated, and that is adhered to the layer 109 by atomic bonding asdescribed more fully below.

The materials selected for the piezoelectric layer 103 can be dividedinto two types: one which has been used for a long time and with a highdegree of freedom in design is used for Rayleigh wave substrates; theother, with less freedom and limited in design, is for Leaky wavesubstrates with low loss characteristics and easily reaches the higherfrequencies by high acoustic velocity, and are mainly used for mobilecommunications. LN and LT materials are often used for broadbandfilters, and according to the filter specifications the manufacturingmaterials and cutting angles differ. Filters for applications thatrequire comparatively low loss mainly generally require Leaky wavematerials, while Rayleigh wave materials are predominately used forcommunication equipment that requires low ripple and low group delaycharacteristics. Among Rayleigh wave materials, ST-cut crystal has thebest temperature characteristics as a piezoelectric material.

In accordance with a representative embodiment, the substrate 108comprises crystalline silicon, which may be polycrystalline ormonocrystalline, having thickness of approximately 100.0 μm toapproximately 800.0 μm. As will become clearer as the presentdescription continues, the material selected for use as the substrate108, among other considerations, is selected for ease of micromaching,using one or more of a variety of known techniques. Accordingly, otherpolycrystalline or monocrystalline materials besides silicon arecontemplated for use as the substrate 108 of the SAW resonator structure100. By way of example, these materials include, but are not limited to,glass, single crystal aluminum oxide (Al₂O₃) (sometimes referred to as“sapphire”), and polycrystalline Al₂O₃, to name a few. In certainrepresentative embodiments, in order to improve the performance of afilter comprising SAW resonator structure(s) 100, the substrate 108 maycomprise a comparatively high-resistivity material. Illustratively, thesubstrate 108 may comprise single crystal silicon that is doped to acomparatively high resistivity.

The layer 109 is illustratively an oxide material, such as SiO₂, orphosphosilicate glass (PSG), borosilicate glass (BSG), a thermally grownoxide, or other material amenable to polishing to a high degree ofsmoothness, as described more fully below. The layer 109 is deposited bya known method, such as chemical vapor deposition (CVD) or plasmaenhanced chemical vapor deposition (PECVD), or may be thermally grown.As described more fully below, the layer 109 is polished to a thicknessin the range of approximately 0.05 μm to approximately 6.0 μm.

The piezoelectric layer 103 has a first surface 110, and a secondsurface 111, which opposed the first surface 110. Similarly, the layer109 has a first surface 112 and a second surface 113. As depicted inFIG. 1B, the first surface 112 of the layer 109 is atomically bonded tothe second surface 111 of the piezoelectric layer 103, as described morefully below.

The substrate 108 has a first surface 114 and a second surface 115opposing the first surface 114. The first surface 114 has a plurality offeatures 116 there-across. As noted above, undesired spurious modes arelaunched in the piezoelectric layer 103, and propagate down to the firstsurface 114. As described more fully below in connection with portion117 in FIG. 1D, the plurality of features 116 reflect undesired spuriousmodes at various angles and over various distances to destructivelyinterfere with the undesired spurious waves in the piezoelectric layer103, and possibly enable a portion of these waves to be beneficiallyconverted into desired SAW waves. Again as described more fully below,the reflections provided by the plurality of features 116 foster areduction in the degree of spurious modes (i.e., standing waves), whichare created by the reflection of acoustic waves at the interface of thesecond surface 111 of the piezoelectric layer 103 and the first surface112 of layer 109. Ultimately, the reflections provided by the pluralityof features 116 serve to improve the performance of devices (e.g.,filters) that comprise a plurality of SAW resonator structures 100.

As noted above, and as described more fully below in connection with thedescription of portion 118 in FIG. 1E, the first surface 112 of layer109 is polished, such as by chemical-mechanical polishing in order toobtain a “mirror” like finish with a comparatively low root-mean-square(RMS) variation of height. This low RMS variation of heightsignificantly improves the contact area between the first surface 112 ofthe layer 109 and the second surface 111 of the piezoelectric layer 103to improve the atomic bonding between the first surface 112 and thesecond surface 111. As is known, the bond strength realized by atomicbonding is directly proportional to the contact area between twosurfaces. As such, improving the flatness/smoothness of the firstsurface 112 fosters an increase in the contact area, thereby improvingthe bond of the layer 109 to the piezoelectric layer 103. As usedherein, the term atomically smooth means sufficiently smooth to providesufficient contact area to provide a sufficiently strong bond strengthbetween the layer 109 and the piezoelectric layer 103, at the interfaceof their first and second surfaces 112, 111, respectively.

FIG. 1D is a cross-sectional view of a portion 117 SAW resonatorstructure 100 according to a representative embodiment. Portion 117 isdepicted in FIG. 1C in magnified view to illustrate various aspects andfunctions of the plurality of features 116 of substrate 108 along theinterface of the first surface 114 of the substrate 108 and the secondsurface 113 of the layer 109.

The shape, dimensions and spacing of the features 116 depends on theirmethod of fabrication. For example, using a known etching technique, theplurality of features 116 are formed in the substrate 108, and may havea generally pyramidal shape 120, with sides 121. Notably, some of theplurality of features 116 may have comparatively “flat” tops 122. Thefeatures 116 also have a height 123 that may be substantially the sameacross the width of the interface between the substrate 108 and thelayer 109. Moreover, the width (x-dimension in the coordinate system ofFIG. 1C) of the features 116 may be the same, or may be different.Generally, however, the width of the features is on the order of thedesired fundamental mode of the SAW resonator structure 100.

Alternatively, and again depending on the method of fabrication, theheight 123 of the features 116 may not be the same. Rather, by selectingthe height 123 of the features to be different, a reduction in theincidence of more than one of the spurious modes can be realized.

The representative method described presently for forming features 116are merely illustrative. Alternative methods, and thus alternative sizesand shapes of the features 116 are contemplated, and some are describedbelow. Notably, regardless of the method used for their fabrication, theplurality of features 116 is beneficially not arranged in a repetitivepattern, and thus are non-periodic. Rather, the plurality of features116 are typically randomly located on the substrate 108, in order toavoid establishing conditions that would support standing waves (i.e.,resonance conditions) in the piezoelectric layer 103, and thereby reducethe incidence of spurious modes in the piezoelectric layer 103.

The substrate 108 is illustratively single-crystal silicon, or othermaterial having crystalline properties. The present teachings make useof the etching properties of the substrate 108 to realize the variouscharacteristics of the features 116. In one representative embodiment,the features 116 are formed by etching the substrate 108 alongcrystalline planes. In this case, the features 116 having pyramidalshapes 120 and sides 121 that are on a “slant” foster reflections atoff-angles relative to the incident direction of the acoustic waves 124.

Turning again to FIG. 1C, acoustic waves 124 are transmitted downwardlyfrom the piezoelectric layer 103, having been generated by the SAWresonator structure 100, and travel through the layer 109. The acousticwaves 124 are incident on one or more of the plurality of features 116,and are reflected therefrom.

As noted above in connection with the description of FIG. 1B, there aremultiple spurious modes, each having a different frequency andwavelength. In accordance with a representative embodiment, the height123 of the features 116 off the substrate 108 is approximatelyone-fourth (¼) λ of one or more of the spurious modes. Selecting theheight 123 of the features to be approximately one-fourth (¼) λ of aparticular spurious mode alters the phase of the reflected waves, andresults in destructive interference by the reflected waves, andsubstantially prevents the establishment of standing waves, and thusspurious modes.

In some embodiments, the height 123 of the features 116 is substantiallythe same, and the height 123 is selected to be approximately one-fourth(¼) λ of one (e.g., a predominant) of the spurious modes. In otherembodiments, the height 123 of the features 116 is not the same, butrather each different height is selected to be approximately equal toone-fourth (¼) λ of one of the multiple spurious modes (e.g., thespurious modes 107 depicted in FIG. 1B). By selecting this one height ormultiple heights, the phase of the reflected waves is altered, andresults in destructive interference by the reflected waves, therebysubstantially preventing the establishment of standing waves of multiplefrequencies, thus preventing the establishment of multiple spuriousmodes.

By way of example, if the spurious modes have a frequency of 700 MHz,the wavelength λ is approximately 6.0 μm. As such, the height 123 wouldapproximately 1.5 μm. By contrast, if the spurious modes have afrequency of 4200 MHz, the λ is approximately 1.0 μm. In this example,the height 123 would be approximately 0.25 μm. More generally, theheight 123 is in the range of less than approximately 0.25 μm (e.g., 0.1μm) to greater than approximately 1.5 μm (e.g., 2.5 μm). As will beappreciated, the range for the height depends on the frequency of thefundamental mode.

The non-periodic orientation of the plurality of features 116, thegenerally, angled surfaces (e.g., side 121) provided by the plurality offeatures 116, and providing the height 123 of the features 116 to be inthe noted range relative to the wavelength of the propagating spuriousmodes combine to alter the phase of the acoustic waves 124 incident onthe various features. Beneficially, these factors in combination resultin comparatively diffuse reflection of the acoustic wave back throughthe layer 109 and into the piezoelectric layer 103. This comparativelydiffuse reflection of the acoustic waves from the features 116 willgenerally not foster constructive interference, and the establishment ofresonance conditions. Accordingly, the plurality of features 116generally prevent the above-noted parasitic acoustic standing waves(i.e., spurious modes) from being established from the acoustic waves124 generated in the piezoelectric layer 103, which travel down and intothe substrate 108.

One measure of the impact of the parasitic spurious modes on theperformance of a device (e.g., filter) comprising a SAW resonator is thequality (Q) factor. For example, the parasitic spurious modes couple atthe interfaces of the piezoelectric layer 103 and remove energyavailable for the desired SAW modes and thereby reduce the Q-factor ofthe resonator device. As is known, the Q-circle of Smith Chart has avalue of unity along its circumferences. The degree of energy loss (andtherefore reduction in Q) is depicted with the reduction of the S₁₁parameter off the unit circle. Notably, as a result of parasiticspurious modes and other acoustic losses, sharp reductions in Q of knowndevices can be observed on a so-called Q-circle of a Smith Chart of theS₁₁ parameter. These sharp reductions in Q-factor are known as “rattles”or “,” and are strongest in the southeast quadrant of the Q-circle.Beneficially, because of the diffuse reflections, and attendant phasemismatch of the reflected acoustic waves 124 realized by the pluralityof features 116, compared to such known devices, a filter comprising SAWresonator structure 100 of representative embodiments of the presentteachings, show lesser magnitudes of the “rattles” or “,” and a somewhat“spreading” of the reduced “rattles” or “ ” is experienced.

As noted above, the plurality of features 116 may be formed by etchingthe substrate 108 to reveal crystalline planes, which thereby form thepyramidal shapes 120. In one embodiment, the substrate is substantiallymonocrystalline silicon that is selectively etched to reveal welldefined crystalline planes with precise orientations. By way of example,selective masking of the substrate 108 having major surfaces lying inthe (110) crystalline plane is initially carried. Illustratively, anetch resistant mask of SiO₂ is patterned using a buffered HF, andetching is effected by the use of an anisotropic etchant such as KOH. Asis known, the depth of the etch is directly proportional to the width ofthe etch in this self-limiting process. This particular orientation ofsubstrate will allow for etching to reveal sides 121 in a certain (e.g.,(111) family of planes.

In one representative method, the etch mask is patterned to provide“dots” in rather random locations over the first surface 114 of thesubstrate 108. After etching, these “dots” are removed, and show theflat tops 122 of certain ones of the plurality of features 116. Thespacing of the “dots” and the duration of the etch, to the self-limitingend, determines the depth of each etch, and therefore, the height 123 ofthe resultant features 116.

Again, the use of monocrystalline silicon for the substrate 108 ismerely illustrative, and other materials can be processed to provide theplurality of features 116 described above.

In other representative embodiments, the plurality of features 116 haverandom spacing, or random orientation, or random heights, or acombination thereof. As can be appreciated, such random spacings,orientations and heights, alone or in combination can fostercomparatively diffuse reflection of the acoustic waves 124 incidentthereon. This diffuse reflection, in turn, alter the phase of theacoustic waves, and serves to reduce the propensity of standing waves(and thus spurious modes) from be established.

The random spacing, orientation, and heights of the plurality offeatures can be effected by a number of methods. For example, theplurality of features 116 may be provided by simply using an unpolishedwafer for the substrate 108. Alternatively, the second surface 115 ofthe substrate 108 could be rough polished by CMP, for example. While theplurality of features 116 of such an embodiment would likely not havethe height relative to the wavelength of the spurious modes, the randomnature of such an unpolished surface would likely provide a usefuldegree of diffusive reflection to avoid the establishment of a resonantcondition for the spurious modes.

FIG. 1E is a cross-sectional view of a portion of a SAW resonatorstructure according to a representative embodiment. Many aspects anddetails of the various features and their methods of fabricationdescribed in connection with the representative embodiments of FIG. 1Eare common to those described above in connection with therepresentative embodiments of FIGS. 1A-1D. Such common aspects anddetails are often not repeated in order to avoid obscuring thedescription of the present representative embodiments.

Notably, the portion depicted in FIG. 1E is somewhat similar to portion117 depicted in FIG. 1D, however differs in the depth of the polishingstep used to provide first surface 112. Specifically, rather thanterminating the polishing of the layer 109 at a height significantlyabove the features 116, the polishing step continues and in placesreveals the features 116. This polishing step thus provides, in places,comparatively “flat” tops 122. By contrast, in other places, thefeatures 116 are not altered by the polishing.

Like the plurality of features 116 depicted in FIG. 1D, the features 116are of the representative embodiments of FIG. 1E are formed by etchingthe substrate 108 along crystalline planes. In this case, the features116 having pyramidal shapes 120 and sides 121 that are on a “slant”foster reflections at off-angles relative to the incident direction ofthe acoustic waves 124. Similarly, like the plurality of features 116 ofFIG. 1D, the height 123 of the plurality of features 116 of therepresentative embodiments of FIG. 1E is approximately one-fourth (¼) λ,of the one or more of the spurious modes. Selecting the height 123 ofthe features to be approximately one-fourth (¼) λ, of a particularspurious mode alters the phase of the reflected waves, and results indestructive interference by the reflected waves, and substantiallyprevents the establishment of standing waves, and thus spurious modes.

In some embodiments, the height 123 of the features 116 is substantiallythe same, and thus the height 123 is selected to be approximatelyone-fourth (¼) λ, of one (e.g., a predominant) spurious modes. In otherembodiments, the height 123 of the features 116 is not the same, butrather each different height is selected to be approximately equal toone-fourth (¼) λ, of one of the multiple spurious modes (e.g., one ofthe spurious modes 107 depicted in FIG. 1B). By selecting such multipleheights, the phase of the reflected waves is altered, and results indestructive interference by the reflected waves, thereby substantiallypreventing the establishment of standing waves of multiple frequencies,thus preventing the establishment of multiple spurious modes.

In other representative embodiments, the plurality of features 116 haverandom spacing, or random orientation, or random heights, or acombination thereof. As can be appreciated, such random spacings,orientations and heights, alone or in combination can fostercomparatively diffuse reflection of the acoustic waves 124 incidentthereon. This diffuse reflection, in turn, alter the phase of theacoustic waves, and serves to reduce the propensity of standing waves(and thus spurious modes) from be established.

The random spacing, orientation, and heights of the plurality offeatures can be effected by a number of methods. For example, theplurality of features 116 may be provided by simply using an unpolishedwafer for the substrate 108. Alternatively, the second surface 115 ofthe substrate 108 could be rough polished by CMP, for example. While theplurality of features 116 of such an embodiment would likely not havethe height relative to the wavelength of the spurious modes, the randomnature of such an unpolished surface would likely provide a usefuldegree of diffusive reflection to avoid the establishment of a resonantcondition for the spurious modes.

FIG. 1F is a cross-sectional view of a portion 118 SAW resonatorstructure 100 according to a representative embodiment. Portion 118 isdepicted in FIG. 1F in magnified view to illustrate various aspects andfunctions of the layer 109 along the interface of the layer 109 and thepiezoelectric layer 103.

In a representative embodiment, layer 109 is PSG, which is depositedover the substrate 108. Illustratively, the PSG is deposited at atemperature of approximately 450° C., using silane and P₂O₅ sources toform a soft glass like material which is approximately 8% phosphorous.This low temperature process is well known to those skilled in the art,and hence, will not be discussed in detail here.

Unfortunately, at the atomic level the surface of such deposited filmsare atomically very rough. However, by polishing the PSG surface toprovide an atomically smooth surface. The surface of the layer 109 isfirst planarized by polishing with a slurry, using a known CMP method.The remaining PSG can then be polished using a more refined slurry.Alternatively, a single more refined slurry can be used for bothpolishing steps if the additional polishing time is not objectionable.As noted above, the goal is to create a “mirror” like finish that isatomically smooth in order to foster strong atomic bonding between thelayer 109 and the piezoelectric layer 103, at the interface of theirfirst and second surfaces 112, 111 respectively.

FIG. 1F depicts four “humps” 125 in the layer after the completion ofthe cleaning of the wafer described above. The “humps” depict variationin the first surface 112. The first hump has a first height, H₁, thesecond hump has a second height, H₂, the third hump has a third height,H₃, and the fourth hump has a fourth height, H₄. For the purposes ofillustration, only four humps are shown. The root mean squared (RMS)variation in the height of the first surface 112 of the layer 109comprised of the four humps depicted is less than approximately 0.5 μm.As noted above, the term atomically smooth herein means sufficientlysmooth to provide sufficient contact area to provide a sufficientlystrong bond strength between the layer 109 and the piezoelectric layer103, at the interface of their first and second surfaces 112, 111,respectively. Such an atomically smooth surface can be realized byproviding the first surface 112 of layer 109 having an RMS variation inheight of in the range of approximately 0.1 Å to approximately 10.0 Å;although beneficially, the RMS variation in height is less thanapproximately 5.0 Å.

As noted above, the forming of an atomically smooth first surface 112.This provides an increased contact area at the interface of the firstand second surfaces 112, 111, respectively, of the layer 109 and thepiezoelectric layer 103. This increased contact area, in turn, fosters acomparatively strong atomic bond between the layer 109 and thepiezoelectric layer 103. Among other benefits, the strong atomic bondbetween the layer 109 and the piezoelectric layer 103 reduces separationor delamination of the layer 109 and the piezoelectric layer 103,thereby increasing the reliability of devices comprising the SAWresonator structure 100 over time.

FIG. 2 is a flow-chart of a method 200 of fabricating a SAW resonatorstructure according to a representative embodiment. Many aspects anddetails of the method, including illustrative materials, processes, anddimensions are common to those described above. These aspects anddetails may not be repeated to avoid obscuring the presently describedrepresentative embodiment.

At 201, the method begins with providing a substrate having a firstsurface and as second surface. As noted above, among other materials,the substrate (e.g., substrate 108) may be monocrystalline silicon.

At 202 a plurality of features (e.g., plurality of features 116) areetched into the substrate.

At 203 a layer is provided over a first surface of the substrate, whichcomprises the plurality of features. As noted above, the layer (e.g.,layer 109) may be an oxide (glass), such as SiO₂ or PSG.

At 204, a first surface of the layer is polished to provide acomparatively smooth first surface. As noted above a CMP method may beused to provide a first surface that is atomically smooth to foster astrong atomic bond between the layer (e.g. layer 109) and thepiezoelectric layer (e.g. piezoelectric layer 103).

At 205, the method comprises atomically bonding a piezoelectric layer tothe first surface of the layer. Notably, comprises contacting thepiezoelectric layer (e.g., piezoelectric layer 103) with the layer thathas been polished to an atomically smooth degree.

As noted above, when connected in a selected topology, a plurality ofSAW resonators can function as an electrical filter. FIG. 3 shows asimplified schematic block diagram of an electrical filter 300 inaccordance with a representative embodiment. The electrical filter 300comprises series SAW resonators 301 and shunt SAW resonators 302. Theseries SAW resonators 301 and shunt SAW resonators 302 may each compriseSAW resonator structures 100 described in connection with therepresentative embodiments of FIGS. 1A-2. As can be appreciated, the SAWresonator structures (e.g., a plurality of SAW resonator structures 100)that comprise the electrical filter 300 may be provided over a commonsubstrate (e.g., substrate 108), or may be a number of individual SAWresonator structures (e.g., SAW resonator structures 100) disposed overmore than one substrate (e.g., more that one substrate 108). Theelectrical filter 300 is commonly referred to as a ladder filter, andmay be used for example in duplexer applications. It is emphasized thatthe topology of the electrical filter 300 is merely illustrative andother topologies are contemplated. Moreover, the acoustic resonators ofthe representative embodiments are contemplated in a variety ofapplications including, but not limited to duplexers.

The various components, materials, structures and parameters areincluded by way of illustration and example only and not in any limitingsense. In view of this disclosure, those skilled in the art canimplement the present teachings in determining their own applicationsand needed components, materials, structures and equipment to implementthese applications, while remaining within the scope of the appendedclaims.

1. A surface acoustic wave (SAW) resonator structure, comprising:substrate having a first surface and a second surface, the first surfacehaving a plurality of features; a piezoelectric layer disposed over thesubstrate, the piezoelectric layer having a first surface and a secondsurface; a plurality of electrodes disposed over the first surface ofthe piezoelectric layer, the plurality of electrodes configured togenerate surface acoustic waves in the piezoelectric layer; and a layerdisposed between the first surface of the substrate and the secondsurface of the piezoelectric layer, the first surface of the layerhaving a smoothness sufficient to foster atomic bonding between thefirst surface of the layer and the second surface of the piezoelectriclayer, wherein the plurality of features reflect acoustic waves andreduce the incidence of spurious modes in the piezoelectric layer.
 2. ASAW resonator structure as claimed in claim 1, wherein the reflectedacoustic waves destructively interfere with acoustic waves in thepiezoelectric layer.
 3. A SAW resonator structure as claimed in claim 1,wherein the features in the first surface of the substrate aresubstantially pyramidal in shape.
 4. A SAW resonator structure asclaimed in claim 3, wherein the features are substantially not in aregular pattern.
 5. A SAW resonator structure as claimed in claim 3,wherein the features have a height of approximately one-fourth of awavelength (¼ λ) of a spurious mode.
 6. A SAW resonator structure asclaimed in claim 3, wherein the features have a height in the range ofapproximately 0.25 μm to approximately 1.5 μm.
 7. A SAW resonatorstructure as claimed in claim 3, wherein the features have a height inthe range of approximately 0.1 μm to approximately 2.50 μm.
 8. A SAWresonator structure as claimed in claim 3, wherein the features have aplurality of heights, and each of the pluralities of heights isapproximately a height in the range of approximately one-fourth of awavelength (¼ λ) of one of the plurality of spurious modes.
 9. A SAWresonator structure as claimed in claim 1, wherein the layer comprisesan oxide material.
 10. A SAW resonator structure as claimed in claim 9,wherein the oxide material comprises silicon dioxide (SiO₂).
 11. A SAWresonator structure as claimed in claim 9, wherein the first surface ofthe layer has a root-mean-square (RMS) variation in height ofapproximately 0.1 Å to approximately 10.0 Å.
 12. A surface acoustic wave(SAW) filter comprising a plurality of SAW resonator structures, one ormore of the plurality of SAW resonator structures comprising: substratehaving a first surface and a second surface, the first surface having aplurality of features; a piezoelectric layer disposed over thesubstrate, the piezoelectric layer having a first surface and a secondsurface; a plurality of electrodes disposed over the first surface ofthe piezoelectric layer, the plurality of electrodes configured togenerate surface acoustic waves in the piezoelectric layer; and a layerdisposed between the first surface of the substrate and the secondsurface of the piezoelectric layer, the first surface of the layerhaving a smoothness sufficient to foster atomic bonding between thefirst surface of the layer and the second surface of the piezoelectriclayer, wherein the plurality of features reflect acoustic waves andreduce the incidence of spurious modes in the piezoelectric layer.
 13. ASAW filter as claimed in claim 13, wherein the reflected acoustic wavesdestructively interfere with acoustic waves in the piezoelectric layer.14. A SAW filter as claimed in claim 13, wherein the features in thefirst surface of the substrate are substantially pyramidal in shape. 15.A SAW filter as claimed in claim 14, wherein the features aresubstantially not in a regular pattern.
 16. A SAW filter as claimed inclaim 14, wherein the features have a height of approximately one-fourthof a wavelength (¼ λ) of a spurious mode.
 17. A SAW filter as claimed inclaim 16, wherein the features have a height in the range ofapproximately 0.25 μm to approximately 1.5 μm.
 18. A SAW filter asclaimed in claim 16, wherein the features have a height in the range ofapproximately 0.1 μm to approximately 2.50 μm.
 19. A SAW filter asclaimed in claim 16, wherein the features have a plurality of heights,and each of the pluralities of heights is approximately a height in therange of approximately one-fourth of a wavelength (¼ λ) of one of theplurality of spurious modes.
 20. A SAW filter as claimed in claim 12,wherein the SAW filter is a ladder filter, comprising a plurality of SAWresonators structures.
 21. A SAW filter as claimed in claim 11, whereinthe layer comprises an oxide material.
 22. A SAW filter as claimed inclaim 21, wherein the oxide material comprises silicon dioxide (SiO₂).23. A SAW filter as claimed in claim 21, wherein the first surface ofthe layer has a root-mean-square (RMS) variation in height ofapproximately 1.0 Å to approximately 10.0 Å or less.
 24. A SAW filter asclaimed in claim 11, wherein two or more of the plurality of resonatorsare configured in a series and shunt configuration.
 25. A method offabricating a surface acoustic wave filter, the method comprising:providing a substrate having a first surface and a second surfaceopposing the first surface; etching a plurality of features in the firstsurface of the substrate; providing a layer over the first surface andthe plurality of features of the substrate, the layer having a firstsurface and a second surface opposing the first surface; polishing thefirst surface of the layer; and atomically bonding a piezoelectric layerto the first surface of the layer.
 26. A method as claimed in claim 25,wherein the layer comprises an oxide material.
 27. A method as claimedin claim 25, wherein the oxide material comprises silicon dioxide(SiO₂).
 28. A method as claimed in claim 25, wherein the first surfaceof the layer has a root-mean-square (RMS) variation in height ofapproximately 0.1 Å to approximately 10.Å.